1. Field of the Invention
The present invention relates generally to ATM (asynchronous transfer mode) communications network, and more specifically to a method of switching communication links without interrupting ATM cell traffic in order to avoid cell loss or duplication of cells. The present invention is particularly suitable for mobile ATM communications networks.
2. Description of the Related Art
In mobile communication systems, digital speech signals of a mobile station are tolerant of short interruptions of a communication link. Such interruptions usually occur as a result of a handoff when the mobile station moves around and crosses the boundary between adjoining cell-site areas.
In the case of conventional ATM communication systems, a handoff protocol is activated when an end-to-end connection is established. However, the protocol takes no account of problems associated with possible switchover of connections during communication. In a proposed wireless ATM system, user data and signaling messages are separately buffered and processed according to individual handoff protocols.
In broadband wireless ATM communications networks, missing or duplication of an ATM cell is detrimental to the quality of end-to-end computer data communication. However, the techniques currently available for use in the broadband ATM network cannot avoid missing and duplication of ATM cells when a handoff is encountered. In addition, the switching speed is low because of the separated buffering of user data and signaling messages.
It is therefore an object of the present invention to provide high speed switching of communication links without interrupting ATM cell traffic.
According to one aspect of the present invention, there is provided a switching method for an ATM communications network, wherein upstream ATM cells and downstream ATM cells are transmitted between a first site and a second site over a first communication link and a request message is sent from the first site to the second site when the first communication link is likely to become unavailable. The method comprises the steps of (a) holding the upstream ATM cells in a first buffer immediately following the transmission of the request message and determining the location of one of the upstream cells within the first buffer which is to be transmitted first when transmission of upstream cells is resumed and storing an address pointer indicating the location in a first address memory, (b) transmitting an end-of-stream OAM (operations, administration and maintenance) cell between the sites over the first communication link, (c) holding the downstream ATM cells in a second buffer in response to the end-of-stream OAM cell and determining the location of one of the downstream cells within the second buffer which is to be transmitted first when transmission of downstream cells is resumed and storing an address pointer indicating the location in a second address memory, (d) establishing a second communication link between the first and second sites, and (e) resuming transmission of upstream ATM cells from the first site over the second communication link, starting with a location of the first buffer specified by the address pointer in the first address memory and resuming transmission of downstream ATM cells from the second site over the second communication link, starting with a location of the second buffer specified by the address pointer in the second address memory.
According to a second aspect, the present invention provides a handoff method for a mobile ATM communications network, wherein upstream ATM cells and downstream ATM cells are transmitted between a mobile site and a fixed site over a first communication link and a handoff request message is sent from the mobile site to the fixed site when the first communication link is likely to become unavailable. The handoff method comprises the steps of (a) holding the upstream ATM cells in a mobile-site buffer immediately following the transmission of the handoff request message and determining the location of one of the upstream cells within the mobile-site buffer which is to be transmitted first when transmission of upstream cells is resumed and storing an address pointer indicating the location in a mobile-site memory, (b) transmitting an end-of-stream OAM cell from the mobile site to the fixed site over the first communication link, (c) holding the downstream ATM cells in a fixed-site buffer in response to the end-of-stream OAM cell and determining the location of one of the downstream cells within the fixed-site buffer which is to be transmitted first when transmission of downstream cells is resumed and storing an address pointer indicating the location in a fixed-site memory, (d) establishing a second communication link between the mobile site and the fixed site, and (e) resuming transmission of upstream ATM cells from the mobile site, starting with a location of the mobile-site buffer specified by the address pointer in the mobile-site memory and resuming transmission of downstream ATM cells from the fixed site, starting with a location of the fixed-site buffer specified by the address pointer in the fixed-site memory.
According to a third aspect, the present invention provides an end user system for an ATM communications network, comprising an interface for establishing a wireless link to the network, a transmit buffer for transmitting upstream ATM cells to the network via the interface, monitor circuitry for monitoring the wireless link and transmitting a request message to the network when the link is likely to become unavailable for informing that the end user system is establishing a new wireless link, an address memory, and a controller for (a) holding the upstream ATM cells in the transmit buffer immediately following the transmission of the request message, (b) determining the location of one of the upstream cells within the transmit buffer which is to be transmitted first when transmission of upstream cells is resumed, (c) storing an address pointer indicating the location in the address memory, (d) transmitting an end-of-stream OAM (operations, administration and maintenance) cell to the network before the new link is established, and (e) resuming transmission of upstream ATM cells when the new wireless link is established, starting with a location of the transmit buffer specified by the address pointer in the first address memory.
According to a fourth aspect, the present invention provides a network apparatus for an ATM communications network, comprising an ATM switching system for establishing a communication link to an end user system via a first base station, a transmit buffer for transmitting downstream ATM cells to the end user system via the ATM switching system, a receive buffer for receiving upstream ATM cells from the end user system via the ATM switching system, a processor for receiving a request message from the end user system when the wireless link is likely to become unavailable and determining a new communication link, an address memory, and a controller for (a) holding the downstream ATM cells in the transmit buffer in response to an end-of-stream OAM (operations, administration and maintenance) cell from the end user system, (b) determining the location of one of the downstream cells within the transmit buffer which is to be transmitted first when transmission of downstream cells is resumed, (c) storing an address pointer indicating the location into the address memory, (d) controlling the ATM switching system to establish the new communication link via a second base station, (e) resuming transmission of downstream ATM cells via the new communication link, starting with a location of the transmit buffer specified by the address pointer in the address memory, and (f) receiving a start-of-stream OAM cell from the end user system via the new wireless link and resuming reception of upstream ATM cells from the end user system by storing received cells in locations of the receive buffer corresponding to the start-of-stream OAM cell.
According to a further aspect, the present invention provides a mobile ATM communications network comprising a network apparatus and a user end system. The network apparatus comprises an ATM switching system for establishing a communication link to an end user system via a first base station, a network transmit buffer for forwarding downstream ATM cells to the ATM switching system, a network receive buffer for receiving upstream ATM cells from the ATM switching system, a processor connected to the ATM switching system for receiving a request message from the end user system and determining a new communication link to the end user system via a second base station, a network address memory, and a controller connected to the ATM switching system for (a) holding the downstream ATM cells in the network transmit buffer in response to an end-of-stream OAM (operations, administration and maintenance) cell from the end user system, (b) determining the location of one of the downstream cells within the network transmit buffer which is to be transmitted first when transmission of downstream cells is resumed, (c) storing an address pointer indicating the location into the address memory, (d) controlling the ATM switching system to establish the new communication link, (e) resuming transmission of downstream ATM cells via the new communication link, starting with a location of the network transmit buffer specified by the address pointer in the network address memory, and (f) receiving a start-of-stream OAM cell from the end user system via the new wireless link and resuming reception of upstream ATM cells from the end user system by storing received cells in locations of the network receive buffer corresponding to the start-of-stream OAM cell. The end user system comprises an interface for selectively establishing a wireless link to one of the first and second base stations, a user transmit buffer for transmitting the upstream ATM cells to the network apparatus via the interface, monitor circuitry for monitoring the wireless link to the first base station and transmitting the request message to the network apparatus for informing that the end user system is establishing a new wireless link to the second base station when the monitored wireless link is likely to become unavailable, a user address memory, and a controller for (a) holding the upstream ATM cells in the user transmit buffer immediately following the transmission of the request message, (b) determining the location of one of the upstream cells within the user transmit buffer which is to be transmitted first when transmission of upstream cells is resumed, (c) storing an address pointer indicating the location in the user address memory, (d) transmitting an end-of-stream OAM cell to the network apparatus before the new wireless link is established, causing the interface to establish the new wireless link, and (e) resuming transmission of upstream ATM cells when the new wireless link is established, starting with a location of the transmit buffer specified by the address pointer in the first address memory.